Re-election trouble is brewing for longtime Republican senators in deep-red states, from South Carolina to Wyoming. And the trouble is from within.

The GOP's restive Tea Party and libertarian wings, energized by their titular leader, Texas Sen. Ted Cruz, and funded in part by starve-government groups like the Club for Growth, are waging 2014 Senate primary challenges in six states — and counting.

Senate Minority Leader Mitch McConnell of Kentucky is under attack, as are Mike Enzi of Wyoming and Lindsey Graham of South Carolina. All were rated among the most conservative in the U.S. Senate last year by the American Conservative Union.

Texas Sen. John Cornyn, the Senate's second-ranking Republican, may join the list of the challenged, but so far he has no viable primary opponent.

As the fallout from the recent government shutdown and default crisis settles, here's our look at the state of play in Senate races that a Republican is likely to win. But just which Republican is the question.